Food producers increasingly offer portioned, prepared foods for their customers' convenience. In these food products, the food can be already prepared, mixed, and seasoned. In some instances, the food is already cooked, leaving the customer to reheat only (if desired). In other cases, the food is provided raw and is cooked by the customer. By moving food preparation from the serving location to a centralized supplier, the customers can achieve better consistency across locations, improved efficiencies of labor and material, and reduced costs. For the food producer, higher profits can be achieved by offering value-added products.
One challenge in providing food in such a manner is the packaging of the food. There are many factors that influence the packaging format and the requirements of the materials used to form that packaging. Some of these requirements are temperatures and pressures seen by the packaging in processing, distribution, and end use; weight, type, and nature of the product; secondary packaging style; marketing considerations such as display surface and consumer perception; costs of both material and processing/packing operations; and regulatory compliance.
For example, prepared single-serve meals are typically sold in rigid trays with a lid (either rigid or flexible) for ease of consumer use in reheating, serving, and eating since the rigid tray can act as both a cooking/reheating vessel and a plate. Material selection for the tray and lid can often be determined by the processing and cooking/reheating requirements. Crystallized polyethylene terephthalate trays (CPET) are commonly used for dual ovenable packages as it can withstand the heat of a conventional or convection oven and irradiation in a microwave oven. Polypropylene (PP) is commonly used for microwave only applications as it is lower cost than CPET and can hold up to microwave irradiation, but not to conventional or convection oven heat. Other common packaging materials such as high density polyethylene (HDPE), polystyrene (PS), high impact polystyrene (HIPS), and amorphous polyesters (e.g. amorphous polyethylene terephthalate (APET); polyethylene terephthalate-glycol modified (PETG); and others, including blends thereof) are not commonly used in cooking or reheating applications due to fitness-for-use and/or regulatory requirements.
Larger format family-size products can be found in a larger variety of packaging formats such as large trays for items like lasagna or form-fill-and-seal (FFS) bags for items like vegetables, french fries, or tater tots. For institutional packaging used in commercial kitchens, the marketing considerations and requirements are simpler, but the cost requirements become more important. For this reason, simpler and quicker operations like FFS bags are often preferred over rigid tray filling and lidding.
Packaging high-moisture content foods and liquids present additional challenges over packaging dry foods in FFS operations, namely in the areas of seal-through-contamination, hot fill, and hermeticity. Some dry foods packaged in FFS (e.g., powders and powder-coated cereals and snacks) can present challenges to seal-through-contamination. However, with liquids, this challenge is even more demanding. With regard to solid foods, the dust and fines limit the effective seal area by blocking the contact between the two sealant surfaces in discrete locations. For liquids, the contamination is instead a thin film spread over the full area of the seal. In some packaging operations, the food can be pasteurized and put in the package while still hot, in a process called “hot fill.” In these cases, the hot tack properties of the sealant can be critical as the fresh seal should maintain its integrity while at an elevated temperature and cooling slowly. Hot tack can also be a critical property for cooking and reheating applications, where internal pressure can build and apply stress on the seals.
An additional challenge to packaging foods and liquids can be hermeticity. When packaging dry foods, hermeticity can be important for shelf life as the food should be protected from oxygen or moisture vapor transmission. However, there is usually a certain level of transmission through incomplete seals or channel leaks in package corners (cross section shown in FIGS. 1b, 1c, 2b, and 2c) or around the center seal area (also called the back seal area or lap seal area in lap seal packages (cross section shown in FIGS. 1b and 1c); or T-zone, fold over seal area, or fin seal area in fin seal packages (cross section shown in FIGS. 2b and 2c)). These very small leaks do not allow any solid product to escape or contamination to enter, but oxygen, moisture vapor, and aromas can diffuse through these channels. For high-moisture content foods and liquid packaging, these minute leaks are unacceptable containment failures. In addition, product can leak out through these channels leading to underweight packages, possibly spoiled product, and possibly damaged secondary packaging.
To understand these obstacles to hermeticity, the forming process must first be understood. The film can be shaped into a tube on a forming collar by bringing the edges of the film together. There are two ways these ends can be joined. The first way is by lap sealing in which the film is first formed into a tube (FIG. 1A) and lap sealed. The end seal is then formed by flattening the package and heat sealing the entire width of the package. A cross section of this is shown in FIGS. 1A-C, where item 10 is the lap seal and item 11 is the inside of the package tube where the product will be filled. In a package formed this way, the most common places for hermetic failures are at the end seal in the corners and the area adjacent to the lap seal. A cross section of a non-hermetic end seal is shown in FIG. 1B, and a hermetic end seal is shown in FIG. 1B. In both figures, item 10 is the lap seal, item 11 is the end seal where the tube has been flattened, item 12 is the area adjacent to the lap seal that is open in FIG. 1B and closed in FIG. 1C, and items 13a and 13b are the corners. These corner failures are typically the result of the restorative spring force caused by the tight folding of the film.
The second way the ends can be joined is a fin or fold-over seal. The film is first formed into a tube (FIG. 2A) and the fin or fold-over seal is made. The end seal is then formed by flattening the package and heat sealing the entire width of the package. A cross section of this is shown in FIGS. 2A-C, where item 10 is the fin or fold-over seal and item 11 is the inside of the tube where the product will be filled. In a package formed this way, the most common places for hermetic failures are at the end seal in the corners and the area adjacent to the fin or fold-over seal. A cross section of a non-hermetic end seal is shown in FIG. 2B, and a hermetic end seal is shown in FIG. 2C. In both figures, item 10 is the fin or fold-over seal, item 11 is the end seal where the tube has been flattened, item 12 is the area adjacent to the lap seal that is open in FIG. 2B and closed in FIG. 2C, and items 13a and 13b are the corners. These corner failures are typically the result of the restorative spring force caused by the tight folding of the film.